The present invention relates to an image forming apparatus and more particularly to an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer comprising an optical scanning device.
Referring to an optical scanning device to be used in an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer, Japanese Patent Publication No. 2002-365582A teaches conditions for correcting the torsion of a scanning beam (a torsion around the central axis of the beam) in a deflection optical system. Moreover, International Patent Publication No. 03/096101 teaches conditions for correcting the curvature of a scanning path.
However, the conditions for correcting the torsion of a scanning beam and the conditions for correcting the curvature of a scanning path are not coincident with each other in many cases. For this reason, it is necessary to properly balance the inclination of a scanning beam with the curvature of a scanning path in order to equilibrate with image forming characteristics and a lens coverage in an actual optical scanning device. In that case, the torsion of the scanning beam is not corrected perfectly. Alternatively, it is also possible to correct the curvature of a scanning path generated by an eccentrically provided or inclined optical face (a lens face or a reflection face) of a scanning optical system with the conditions taught in Japanese Patent Publication No. 2002-365582A. Also in that case, the torsion is generated on the beam.
When a scanning line obtained by the scanning optical system is included and a perpendicular plane to the optical axis of the scanning optical system is defined to be a virtual scanned face, the inclination of a beam spot formed on the virtual scanned face is more increased apart from a scanning center in a primary scanning direction when the torsion of the scanning beam is not corrected perfectly.
In the deflection optical system disclosed in the publications, incidence is carried out twice over a reflection face.
FIG. 12 shows such an optical system in which a deflector is constituted by a polygon mirror 10 having a plurality of reflection faces 11 (six faces in the drawing) on the side portion of a polyhedral cylinder and the reflection faces 11 are rotated around a rotary axis 12. Two fixed flat mirrors 13 and 14 are disposed so as to face one of the reflection faces 11 subjected to the light deflection. The fixed flat mirrors 13 and 14 are angled relative to each other while forming a gap 15 therebetween.
A light emitted from a light source 21 is converted into a parallel beam a0 through a lens 22. In a case where it is adopted an optical system for correcting a pyramidal angle error of the polygon mirror 10, the light beam emitted from the light source 21 is converted into a parallel light beam relative to a direction perpendicular to the rotation axis 12 of the polygon mirror 10 while being converted into a light beam to be focused in the vicinity of the reflection face 11 of the polygon mirror 10 relative to a direction parallel to the rotation axis 12 of the polygon mirror 10. The light beam a0 passes through the gap 15 between the fixed flat mirrors 13 and 14 and is incident on the reflection face 11 from obliquely below. A light beam a1 obtained by a first reflection through the reflection face 11 advances obliquely upward and is incident on the fixed flat mirror 13. A light beam a2 reflected by the fixed flat mirror 13 advances downward and is incident on the fixed flat mirror 14. A light beam a3 reflected by the fixed flat mirror 14 is incident on the reflection face 11 again. A light beam a4 obtained by a second reflection through the reflection face 11 passes through the gap 15 and advances obliquely upward, and is converted into a focused light beam through a scanning optical system 23 to be incident on a scanned face 24.
Since the reflection face 11 is rotated around the rotary axis 12, the focused light beam is moved at a rotating speed which is approximately four times as high as the rotating speed of the reflection face 11 to draw a scanning line b on the scanned face 24. With the rotation of the polygon mirror 10, the adjacent reflection faces 11 are sequentially subjected to the incident light beam a0. With the rotation of the polygon mirror 10, therefore, the scanning line b is sequentially drawn from one of ends to the other end in the same position on the scanned face 24. A member to be scanned on the scanned face 24 is moved at a constant speed in a perpendicular direction to the scanning line b, resulting in the execution of a raster scan in which the scanning line b is arranged at a constant pitch over the scanned member. When a plane including the central beam of the incident light beam a0 and parallel with the rotary axis 12 is defined as an incident plane, the two fixed flat mirrors 13 and 14 are provided perpendicularly to the incident plane.
It is apparent from the description in connection with FIGS. 19 to 21 in Japanese Patent Publication No. 2002-365582A that the same inclination of a beam spot is generated in a case where an optical system having an optical face provided eccentrically in a secondary scanning direction relative to an optical axis is used in place of the deflection optical system shown in FIG. 12.
In general, the scanned face (photosensitive face) of an electrophotographic printer is inclined relative to the optical axis of an optical scanning device such that the scanned face is rotated about a scanning line as a rotation axis. Such a provision is carried out in order to prevent a light beam reflected by the scanned face from returning to the optical scanning device to cause an unstable laser oscillation, or from being reflected by another optical component in the optical scanning device, thereby becoming ghost light irradiating the scanned face again. Moreover, an incident angle in the primary scanning direction of an incident beam is more increased apart from a scanning center, and the incidence is carried out with an angle in the primary and secondary scanning directions at the scanned face other than the scanning center. For example, even if there is no inclination of a beam spot over the virtual scanned face, the inclination of the beam spot is generated if the scanned face is provided with an inclination in the first and secondary scanning directions relative to the optical axis of the optical scanning device.
Based on a positional relationship between the optical scanning device and the scanned face, the inclinations of the beam spot which are caused by the two factors are combined. Consequently, a fluctuation in characteristics of a beam spot formed on the scanned face within the scanning range is more increased.